Pre-exposure prophylaxis (PrEP) dramatically reduces the risk of HIV infection, especially for those who are most adherent. However, for PrEP to significantly impact the HIV epidemic, those most at risk of HIV must both use and adhere to PrEP. Data suggest that a substantial proportion of men who have sex with men (MSM) stop using PrEP soon after starting; they may or may not resume use. There is also evidence that many young men may not consistently adhere to PrEP. However, we know very little about the frequency of PrEP termination, gaps in PrEP use, or lapses in prevention-effective adherence among young Black MSM (YBMSM), or about short- and long-term changes in use and adherence for these men. Additionally, we lack information about how use and adherence among YBMSM may be impacted by structural and psychosocial barriers to PrEP (including issues with health care access and insurance; low socioeconomic status and instability in employment, transportation, and housing; poor mental health; substance use; and stigma and negative social norms related to PrEP) or partner and relationship factors (such as the introduction of new partners, the development of committed relationships, perceptions of partner risk, and intimate partner violence). The overall aim of the proposed research is to use short- and long-term prospective, longitudinal data from YBMSM PrEP users to examine patterns and predictors of PrEP use and adherence, with the goal of identifying targets for PrEP interventions with this high-risk population. We will recruit 200 YBMSM (ages 18- 30) who use PrEP from Midwestern metro areas with large racial disparities in HIV (Detroit, MI; Kansas City, MO; Milwaukee, WI; and Minneapolis, MN). Men will complete surveys assessing PrEP use and adherence along with hypothesized predictors every 3 months for 1 year. Additionally, a subsample of 100 users will complete 14 days of electronic daily diaries to assess micro changes in adherence, other sexual health and risk behaviors, and key predictors. Dried blood spot tests for tenofovir will be used to objectively assess adherence, and information on HIV and STI testing and infection will be drawn from linked medical records. Through the application of modern analytic methods, this research will address the following aims: (1) Describe longitudinal changes in PrEP use and adherence among YBMSM, and associations between PrEP use and adherence and other sexual health and risk behaviors and outcomes; (2) Identify factors associated with gaps in PrEP use, PrEP termination, and poor PrEP adherence for YBMSM over 1 year; and (3) Identify factors associated with PrEP adherence at the daily level for YBMSM using daily diary surveys. We predict that gaps in PrEP use, PrEP termination, and lapses in prevention-effective adherence will be common, and that they will be associated with structural and psychosocial barriers as well as sexual partner and relationship factors. Information on how and why use and adherence vary over time will inform the development of interventions targeting PrEP uptake and adherence among YBMSM.